In certain applications, it is important to detect limit levels in order, for example, to prevent the container in which a filling material is stored from overflowing or running dry. Examples for which it is advantageous to detect a predefined fill level are process tanks, storage tanks, silos or pipelines in the process manufacturing industry. Limit level switches, which can also be referred to as limit switches, limit level detectors or limit level measuring instruments, are used for this purpose. Devices of this type can be used in a wide variety of liquids and granulated or pulverised bulk materials.
Different limit level switches are used depending on the property of the filling material and on the individual process conditions. For example, TDR (time-domain reflectometry) detectors, vibration limit switches and sensors which operate in accordance with the capacitive measurement principle are known. A switching command generated by the limit level switch either starts or stops filling apparatuses, such as belt conveyors or pumps, for example.
The measurement signal detected by the limit level switch changes depending on whether or not the measuring probe of the limit level switch is surrounded by filling material. The measurement signal can also depend on whether the measuring probe is clean or whether it is covered in a build-up of deposits (dirt). Complex evaluation methods are often used in order to reliably evaluate the detected measurement signal.